The present invention relates to improvements in the continuous monitoring of an element in a molten substance containing same, by monitoring the electromotive force (emf) generated between the substance and a reference material. More particularly, the invention is concerned with an improved electrochemical probe for use as reference electrode in carrying out such monitoring.
During the refining of aluminum, the sodium content of molten aluminum must be reduced to the order of a few parts per million, particularly if thin sheets are to be rolled. At present, the Na content is monitored by spectroscopic analysis of samples. It would be advantageous to develop a so-called "sodium-probe" suitable for industrial application and which could be placed in the molten aluminum to give an immediate and continuous reading of the Na content. Such a probe would also be useful in determining the Na content of molten Al-Si alloys to which Na in concentrations of the order of 100 ppm has been added as a structural modifier.
Similarly, lithium must also be removed from molten Al during refining, and an analogous "Li-probe" for the continuous monitoring of Li contents in the ppm range would be an asset. Such a probe would also be very useful in measuring the Li content of molten Al-Li alloys in which the Li concentration is of the order to a few weight %.
Fray has already proposed in British Pat. No. 1,470,558 a solid electrolyte probe based on .beta.-alumina for measuring the Na concentration in molten Al. The reference material used by Fray is solid and comprises a mixture of .alpha.-alumina and .beta.-alumina in equilibrium with the atmospheric air. Although this system responds to the presence of Na in molten Al, the experimental results obtained do not agree with Nernst's Law since the sodium concentration varies in linear manner with the emf generated between the electrodes. It would appear that since the equilibrium reaction occurs in solid phase, the speed of reaction is slow so that the reference electrode potential is unstable. The electrodes were also found to be quite polarizable and very sensitive to humidity absorbed on the electrolyte surface. As a result, such a Na-probe lacks precision and reproducibility.
Fray later proposed in British Pat. No. 1,602,564 an improved version of the above Na-probe, consisting of an airtight probe in which the reference material comprises a mixture of .alpha.-alumina, .beta.-alumina and a mixture of a metal and metal oxide such as a Cu/CuO.sub.2, Cr/Cr.sub.2 O.sub.3 or Ni/NiO mixture which provides a fixed oxygen potential within the sealed probe. However, the experimental results obtained still showed polarizability of the electrodes and did not have a reproducibility sufficient for enabling such probes to be useful in the metallurgical industry.